General Information
 

Salts are amenable to analysis by FAST ATOM BOMBARDMENT (FAB) and ELECTROSPRAY (ES).  Ionization polarity will depend upon the nature of the salt.  If the cation is the species of interest then positive ion mode should be requested.  Anions will be better observed in negative ion mode.
 
 

When to use FAB

FAB is particularly good at greasy molecules.  For example, alkyl ammonium salts are particularly amenable to positive ion FAB analysis, while alkyl sulfates and phosphates work well in negative ion FAB.  A further benefit of FAB for this type of sample is that the samples are less prone to contaminate the FAB instrument than the Electrospray instrument.
 

When to use Electrospray

Electrospray is appropriate for more hydrophilic salts.  Electrospray is usually run in a fairly aqueous environment, so greasy salts tend to stick around, contaminate successive samples and generally clog things up.  You will notice that the Electrospray submission form requests information on the greasy salt content  (NBu₄⁺  and PPh₄⁺) of your samples.  There is a good reason for this request !